Surface fluorinated hydrogen containing material and process for making

ABSTRACT

A MANUFACTURED MATERIAL AND A PROCESS FOR MAKING THE MATERIAL IS DISCLOSED. THE MANUFACTURED MATERIAL I A HYDROGEN CONTAINING SUBSTANCE HAVING A FUORINATED SURFACE AND IN WHICH HYDROGEN ATOMS, WHICH MAY BE PRESENT IN HYDROXYL RADICALS ATTACHED TO A CARBON ATOM CHAIN, HAVE BEEN REPLACED IN THE SURFACE OF THE MATERIAL BY FLUORINE ATOMS OR RADICALS, AND WITH SUCH FUORINATED SURFACE OF THE MATERIAL CONTAINING AT LEAST ABOUT 2 MICROGRAMS OF SUCH FLUORINE PER CM.2 OF SURFACE AREA. THE PROCESS FOR MAKING SUCH A MATERIAL INCLUDES THE STEPS OF SELECTING A SUBSTRATE CONTAINING HYDROGEN ATOMS WHICH MAY BE PRESENT IN HYDROXYL RADICALS ATTACHED TO A CARBON ATOM CHAIN, SELECTING A GAS CONTAINING FLUORINE ATOMS OR RADICALS, PLLACING THE SELECTED SUBSTRATE BETWEEN ELECTRODES IN A FLOWING ATMOSPHERE OF THE SELECTED GAS AT SUBATMOSPHERIC PRESSURE, AND SUBJECTING THE SUBSTRATE TO AN ELECTRODELESS DISCHARGE OF AT LEAST ABOUT 0.2 K.W.H/YARD 2 TO CHEMICALLY ACTIVATE BOTH THE SURFACE OF THE SUBSTRATE AND THE GAS, AND EXCHANGE THE FLUORINE ATOMS OR RADICALS FOR SURFAE HYDROGEN ATOMS OR RADICALS TO PRODUCE A MATERIAL WITH A SURFACE. WHICH COMPARED T THE SUBSTRATE BEFORE THIS PROCESS TREATMENT, IS MORE WATER REPELLENT (WITHOUT SEALING PORES), IS MORE CORROSION AND SOIL RESISTANT, MORE CHEMICALLY INERT, MORE LIKE THE RELATVELY EXPESIVE POLYTETRAFLUOROETHYLENE. WHEN THE SUBSTRATE IS A MATERIAL HAVING A RELATVELY INERT SURFACE, SUCH AS POLYETHYLENE, THE PROCESS IS DISCLOSED AS INCLUDING AN INITIAL TREATMENT STEP THAT INVOLVES ION BOM-BARDMENT OF THE SUBSTRATE IN HELIUM GAS TO ACTIVATE THE SURFACE BEFORE THE SURFACE IS TREATED IN THE FLUORINE CONTAINING GAS.

June 19, 1 973 J. P. MANION ETAL SURFACE FLUORINATED HYDROGEN CONTAINING MATERIAL AND PROCESS FOR MAKING 2 Shets-Sheet 1 Of'iginal Filed Aug. 1. 1969 7 a OM" Y m x0 5/ m WHO m m w 8 9 o. 8 1& I O o 6 3 4 5 w w 6 B N w W LWM O R MAL n m a a 2 m E N WW Nw m M E W mwm M m H m mm m m M M R W Q m w S G MmR w &% u l? W m 3 GAS INLET WEIGHT L0ss(MILuERAMs) June 19, 1973 J. P. MANION ErAL 3,740,325

SURFACE FLUORINATED HYDROGEN CONTAINING MATERIAL AND PROCESS FOR MAKING Original Filed Aug. 1. 1969 2 Sheets-Sheet 2 IZOP o o E5 3 so Io 2;

(I o 40 DllSfiHARGED TREATED :1 TEFLON EIUNTREATED FILM OTREATED POLYETHYLENE 20 a uNrREATED POLYETHYLENE l I l l I O l l l' I l I. 0|"23455 O8l624324048 EXPOSURE TIME (HOURS) AT 75c SURFACE AREA(CM2) 600- gsoo O! Q /)(/X/ G g 300 8 Lu 200 Ct g I00- l 0 l I I l I I I I I DISCHARGE TREATMENT TIME (sEcoNDs) United States Patent U.S. Cl. 204-169 6 Claims ABSTRACT OF THE DISCLOSURE A manufactured material and a process for making the material is disclosed. The manufactured material is a hydrogen containing substance having a fluorinated surface and in which hydrogen atoms, which may be present in hydroxyl radicals attached to a carbon atom chain, have been replaced in the surface of the material by fluorine atoms or radicals, and with such fluorinated surface of the material containing at least about 2 micrograms of such fluorine per cm. of surface area. The process for making such a material includes the steps of selecting a substrate containing hydrogen atoms which may be present in hydroxyl radicals attached to a carbon atom chain, selecting a gas containing fluorine atoms or radicals, placing the selected substrate between electrodes in a flowing atmosphere of the selected gas at subatmospheric pressure, and subjecting the substrate to an electrodeless discharge of at least about 0.2 k.w.h./yard to chemically activate both the surface of the substrate and the gas, and exchange the fluorine atoms or radicals for surface hydrogen atoms or radicals to produce a material with a surface, which compared to the substrate before this process treatment, is more water repellent (without sealing pores), is more corrosion and soil resistant, more chemically inert, more like the relatively expensive polytetrafluoroethylene. When the substrate is a material having a relatively inert surface, such as polyethylene, the process is disclosed as including an initial treatment step that involves ion bombardment of the substrate in helium gas to activate the surface before the surface is treated in the fluorine containing gas.

This is a division of application Ser. No. 846,767 filed Aug. 1, 1969, now U.S. Pat. 3,674,667.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a product and a process for making the product, utilizing an electrical discharge to improve certain desired surface characteristics of certain nonmetallic materials and in particular for improving the water repellency of textiles and paper materials and corrosion resistance of polymeric materials; while maintaining unchanged such characteristics of the bulk of the material as the mechanical strength, the dielectric strength, and the resistance to thermal degradation. The field to which the present invention relates therefore encompasses treatments for providing a material with a combination of properties, i.e. properties associated with the surface and properties associated with the bulk of the material, not characteristic of the material before treatment.

Description of the prior art Although a wide variety of polymeric materials are available, both natural and synthetic, the desired combination of properties (at least at prices required for practical commercial applications) is frequently not found in 3,740,325 Patented June 19 1973 any one material. Formulations involving mixtures of compatible material with filler are frequently resorted to but with only limited success.

For the special need for materials having such as the previously mentioned desired surface properties combined with desired properties characteristic of the bulk of the material, graft polymerization has been used to apply a surface graft having the desired surface properties, on to a base of a polymeric material having other desired properties. According to the prior art this may be accomplished in either of two ways. One way is to activate a polymer surface with high energy radiation or an RF discharge in the presence of an eligible monomer as described in U.S. Pat. No. 3,068,510 to Coleman. A second approach known to the prior art involves the use of a polymer made under conditions which preserve an active site for the later graft reaction. In the first approach the radiation level required for significant surface treatment results in excessive cross-linking of the base material and subsequent loss ofdesirable bulk properties. The second approach is limited in application to special systems such as condensation polymers. 2

Graft polymerization, by whatever prior art approach it is achieved, involves a disadvantage when applied for the purpose of making such as a tenting material or wearing apparel water repellent. The disadvantage of graft polymerization is a result of complete surface sealing, including pores, which eliminates the property of air breathing so important to providing a comfortable rain repellent apparel or tenting material.

It is also known to the prior art that surfaces of natural and synthetic fibers (woven or nonwoven) may be treated with an electrical discharge in certain gaseous mediums to provide increased adhesion and wetting properties important to providinga surface which, for example, adheres to other materials or provides improved ink and dye retention, resistance to corona degradation, slip and antistatic properties. Adhesion, however, is a characteristic that may be viewed as being the opposite of water repellency. The gases used in these prior art practices to increase adhesive properties are inoperative to increase water repellency and corrosion resistance of materials so treated, which is sought and achieved, as will be later explained, by the present invention. Certain U.S. patents may be identified which are examples of this prior art approach (i.e., treating a surface to increase its adhereability and wettability rather than applying a graft by either of the heretofore described graft approaches). For example, the prior art described in U.S. Pat. No. 3,291,712 to McBride discloses that perfluorocarbon polymers are very resistant to corrosion and the action of most chemicals and solvents. The McBride patent further states that perfluorocarbon polymers are therefore used to line pipes and vessels used to transport or store chemicals but that perfluorocarbon polymers do not adhere Well to such surfaces. This McBride patent also discloses treating the surface of a perfluoro carbon polymer web with an electrical discharge in the presence of an amine vapor to render the perfluorocarbon Web strongly adhereable to other materials so it can be easily applied to and will adhere well to pipes, vessels and the like.

Another example of a prior art patent related to treating polymeric-shaped structures to increase their adhereability to other materials and their wettability by other materials (i.e., ink, dye, etc.), is U.S. Pat. No. 3,255,099 to Wolinski. This Wolinski patent discloses achieving such objects by treating polymeric-shaped structures with an 1 Atomic Radiation and Polymers, A. Sharlesby, Pergamon Press (1960), p. 498 et seq.

* Chapiro, A., J. Polymer Set. 23, 377 (1957).

electrical discharge in the presence of a vapor of an inorganic agent having an aflinity for electrons.

Still another example of a prior art patent related to treating polymeric-shaped structures to improve its adhereability and in this case also its heat scalability, is another U.S. patent to Wolinski, No. 3,255,089. This Wolinski patent discloses treating the surface of polymeric-shaped structures with an electrical discharge in the presence of an organic compound selected from the group consisting of polymerizable organic compounds, nonpolymerizable compounds having replaceable hydrogen atoms, and perhalohydrocarbons having a bond disassociation energy for the carbon halogen bond below 100 kilocalories. The stipulation that perhalohydrocarbons having a carbon-halogen bond below 100 kilocalories, in order to be within the scope of Wolinskis teachings, makes it completely clear to those skilled in this art, among whom the present inventors number themselves, that the Wolinski teaching excludes fluorine-containing perhalohydrocarbons such as are involved in the present invention, in different mechanisms and for different purposes, as will appear from the description to follow.

SUMMARY OF THE INVENTION It is the object of the present invention to provide new and improved materials by a process for upgrading the value of certain low cost materials to thus providing new and more useful materials, by treating the surface thereof to impart properties to the surface that are equal to or superior to properties of relatively high cost materials.

Another object of the present invention is to provide an inexpensive process for upgrading the value of certain already relatively expensive materials by treating the surface thereof to further improve desired surface properties that are already possessed by and characteristic of such materials, and thereby increase the economic justification for using such expensive materials.

More specific objects of the present invention include providing a process for treating materials to improve the surface properties such as water repellency to make such materials more useful and comfortable for such as tenting and rain repelling apparel; and for treating materials to improve surface properties such as corrosion resistance to make such materials more useful for such as coating or wrapping wires and cables for underground burial.

These and other objects that are achieved in a manner that will be described, involve attaining goals which are the exact opposite of increasing adhereability, wettability, etc. which are the goals of the described prior art. It is in the nature of a discovery by the present inventors that the achievement of the objects of this invention requires a material to be treated which has in its surface hydrogen atoms which may be present in hydroxyl groups attached to a carbon atom chain, and a treating gas containing fluorine atoms or fluorine containing radicals. As will appear as the description of this invention proceeds, this invention does not involve graft polymerization (e.g., Coleman) nor does this invention involve treating a surface to make it more adhereable to other materials (e.g., Wolinski). This invention does, however, involve an electrical discharge being utilized to chemically activate both a surface containing hydrogen atoms or hydrogen radicals and a treating gas containing fluorine atoms or fluorine radicals (with gaseous perfluoro compounds proving most satisfactory) resulting in an exchange of the fluorine atoms or radical for the hydrogen atom or radical, and the production of a surface that is more water repellent (without sealing pores), more corrosion and soil resistant, more chemically-inert and more like the relatively expensive polytetrafluoroethylene which is known by the Du Pont trademark Teflow, than was the surface before such treatment.

Materials which can be used as a substrate to be treated by the present invention, and which possess hydrogen atoms with or without hydroxyl radicals, attached to a carbon chain molecule include such as cotton, wool, rayon, paper and the cellulosic polymers, polyethylene, polypropylene, and the polyolefins, nylon, Dacron, Orlon (Dacron and Orlon are DuPont trademarks), and the polyesters, polyvinyl fluoride and the polyvinyl and polyacrylic polymers, epoxy, phenol formaldehyde, polyurethane, and a great many other such materials.

Treating gases containing fluorine atoms or radicals, operative with the present invention include the following perfluoro compounds CF C 1 C 1 C F CHF and SP For treating some materials, for example polyethylene, a preferred process according to the present invention may include a pretreatment in helium gas to cleanup and activate the polyethylene surface for better treatment thereafter by the fluorine atom containing gas.

TESTS AND MEASUREMENTS In the description of the invention to follow many examples will be described and evaluated relative to each other and control (untreated) samples of the same materials treated according to this invention. In order to present such evaluations certain tests and measurements were used which will now be described.

Water repellency Two tests for water repellency were used. One test was a static water drop test. This test consisted of placing drops of distilled water on a fabric surface to be tested using a spacing of approximately one drop per square inch. Delivery of the drops to the fabric surface was from a standard pipette to assure reasonable reproducibility of size. Water repellency rating was taken to be the time after drop addition to the first sign of fabric wetting, penetration or wicking for any drop. The untreated control samples of cotton or paper tested absorbed a water drop completely in 13 seconds. The untreated control samples of synthetic cloths absorbed the drop completely in 1-5 minutes. Typically, a cloth, natural or synthetic, or paper, treated according to the present invention, maintained 28 drops for 24 hours (a 28 square inch sample was usually tested).

A second test conducted for water repellency was the ASTM D-58363 static immersion test (1968 Book of ASTM StandardsPart 24Textile Materials, American Society for Testing and Materials, Philadelphia, Pa.). The samples, after conditioning at a relative humidity of 65%, were weighed, immersed in water at an average hydrostatic head of 3.5 inches for 20 minutes at ambient temperature. The samples were then placed between two pieces of dry blotting paper and passed through a wringer having a constant load to remove interstitial water and reweighed. The percentage weight gain is taken to be due to the water absorbed.

Tensile strength Tensile strength of both dry and wet kraft paper was measured. One-half inch strips, 5 mils thick and eight inches in length were cut from treated paper and from untreated paper and the tensile strengths measured with an Instron Model TTC Universal Testing apparatus. The use of a liquid immersion test attachment permitted wet strength tests to be performed with the water level several inches above the upper Instron jaw.

Fluorine analysis Polyethylene film, cotton cloth and kraft paper samples were analyzed for fluoride content using a micromethod of Yamamura et al. (Analytical Chemistry, volume 34, page 1308 of 1962) with minor modifications. Lanthanum nitrate was substituted for cerium nitrate, the complexing agent. It was also necessary to ignite the samples wrapped in a platinum screen in an oxygen-filled, taped, quartz flask containing water to obtain the fluoride in watersoluble form. The method, as modified, was calibrated with known samples prepared from a standard sodium fluoride stock solution.

Corrosion measurements The resistance of treated polyethylene surfaces to accelerated oxidative attack was determined by the immersion of treated samples according to the present invention in concentrated anhydrous chromic acid solution (150 milliliters conc. ,H SO /40 grams of Na Cr O at 75 C. and measuring their weight loss. Treated samples, together with an untreated control sample of polyethylene and a similar sample of polytetrafluoroethylene were oven-dried and weighed before and after various times of acid contact.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic illustration of an apparatus operative to demonstrate the present invention;

FIG. 2 is a side view of the electrode arrangement in apparatus shown in FIG. 1;

FIG. 3 is a diagrammatic illustration of an early model of a batch apparatus operative to practice the present invention;

FIG. 4 is a graph depicting corrosion resistance of materials;

FIG. 5 is a graph depicting fluorine content in the surface of materials; and

FIG. 6 is a graph depicting the effect of exposure time of a material to a treatment according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, a device for practicing and demonstrating the present invention includes a chamber 1 containing a supply roll 2 and a take-up roll '3 with a span of substrate material 4 extending therebetween through an inner chamber 5. The inner chamber 5 is provided with guide slits 6, 7 aligned to position a midportion of span 4 between an upper array of electrodes 8 and a lower array of electrodes 9. The electrodes 8, 9 are supported at their end portions by support walls 10, 11. Suitable means ((not shown) may be applied to roll 3 to rotate roll 3 and draw material through the inner chamber 5. The inner chamber 5 is also provided with an evacuation port 12 for connection to a standard pump (not shown) such as a Welch Model 1397. The chamber 5 is also provided with a treatment gas inlet 14 for connection to gas supply sources (not shown). As shown in FIG. 2, the support walls 10, 11 are provided with gas flow holes 18 in wall below the span of material 4 and gas flow holes 19 above span 4 in wall 11. Gas admitted at 14 thus passes over and beneath the material 4 before being drawn out at 12. The gases used in the examples which will be described, were of commercial grades of CR (95-98%); CH-F (98%); CgFa (99.6%); C F (99.99%); CClgFz (99%); SE; (90--' 94%); N (98%) and helium (99.9%) produced by the Matheson Company Inc., and such gases with no further purification were used at the selected pressure, measured at a port 16 at extremely low flow rates, e.g., 5 millimoles of gas per minute.

Each electrode comprises a glass or metal tube 20, which may be for example a tube of borosilicate glass of 6 mm. outside diameter, and contains a filament 21 which may be a wire, a roll of metal foil or an electroplated metallic deposit on the inner surface of the tube. Each electrode of the upper electrode array 8 is connected electrically in parallel as were the electrodes of the lower array 9. Power was connected at electrodes 8a and 9a. The power source used (not shown) was a Lepel Model T2.51-KC-M-S high frequency induction heating unit which was modified to drive an air core, high frequency transformer for the production of high voltage in the 200 to 500 kc. frequency range. Electrical power dissipated in the chamber 1 was determined with a tetronix oscilloscope, Model 536, equipped with an electronic switch. Discharge voltage and current were observed simultaneously and the phase angle and frequency measured. Because the filament 21 is within such as a glass tube, the discharge is such as is known to those skilled in related arts as an electrodeless discharge.

Referring to FIG. 3, an apparatus is shown that is an early model of a batch apparatus used to treat materials in tubular form. This apparatus has a cylindrical cell 30 with a gas port 31 and a gas outlet port 32 for connection to a vacuum system (not shown). Openings 33, 34 are provided in cell 30 perpendicular to the gas inlet port 31 and gas outlet port 32. A metal electrode 35 is arranged within a closed end glass insulation tube 36 which projects through opening 33 into cell 30. A tubular sample 37 of material to be treated is arranged over the glass tube 36 to be centrally located within cell 30. A ring seal 38 is provided to prevent treatment gas escaping around the periphery of tube 36. A tantalum wire spiral electrode 40 projects through opening 34 and is arranged to present the plane of the spiral 41 perpendicular to a central axis through cylindrical cell 30. The wire electrode 40 is drawn through a seal 42 inserted in opening 34. The power source is a Cenco Tesla Coil, catalog number 80700, driven by 15 kv., 60 milliampere Neon sign transformer and spark gap. Maximum primary circuit power was 50 watts. Although load output was not determined, the known inefficiency of this power unit accounts for the time required for effective treatment of polyethylene. As will be shown, treatment time and treatment power are inversely related. Treatment time can be reduced with increase in power and more efficient use of the power supplied than was possible with the Tesla unit used.

The present invention as practiced with either the apparatus of FIGS. 1, 2 or FIG. 3, provides a general polymer surface treatment technique which renders polymer film and solid surfaces (as opposed to porous or woven) more chemically-inert like Teflon and which produces an equally chemically-inert surface on porous fibers, po-

rous paper, wood and woven textiles, natural and synthetic, rendering them water repellent without change in their porosity or air-breathing characteristics.

In general, surface treatment of the material to be treated, is accomplished by subjecting the material to an electrodeless discharge in an atmosphere of gas within chamber, in which the gas molecules contain fluorine atoms. As will appear from the description of examples to follow gaseous perfluorohydrocarbons have been found to yield the best results with respect to treatment efficiency and absolute treatment achieved.

With the apparatus of FIG. 1, woven textile, polymer or paper sheets are drawn through slit 6, between the upper and lower electrode arrays 8, 9 in chamber 5 and through slit 7 to take-up roll 3. The vertical separation between the upper and lower electrode arrays 8, 9 has been varied and a separation of 0.5 inch between upper and lower electrode centers has been used to advantage with 6 mm. O.D. glass tube electrodes arranged in the arrays 8, 9. As stated previously, each glass tube contains a conducting filament 21 and the upper electrodes 8 are connected electrically in parallel as are the lower electrodes 9. An advantage for the electrode array shown in FIG. 1 is that both sides of the span 4 of film, paper or textile fabric are treated equally and, of course, simultaneously.

PROCESS VARIABLES Gas flow rates and condition At the discharge wattages used the lowest gas flow rates measured were found to yield adequate exchange and in examples to be described, was approximately 5 millimoles of OR; per minute at a pressure of 5 torr. For continuous operation over extended periods a small recycle pump (not shown) preceded by a chemical absorption chamber (not shown) may be used. The chemical absorption chamber can contain an alkaline substance such as sodium hydroxide or other alkaline material having low vapor pressure to react with the hydrofluoric acid and is followed by a section of drying agent such as 8-14 mesh activated alumina to insure the treatment gas being returned to the treatment chamber 1 as dry as possible. As will be shown later, the efiiciency of paper treatment is decreased significantly by the presence of residual moisture in the paper.

The required pressure of the treatment gas has been found to vary with the type of surface being treated. Thus, although acceptable treatment of the textile fabrics, natural and synthetic, are obtained over a wide range of subatmospheric pressure, optimized treatment occurs over a moderately narrow pressure range around torr, that is, between about 0.5 and 50 torr. In the treatment of kraft paper eifective treatment is achieved at pressures over the range 0.5 to 400 torr. In general it has been found that the pressure range in which treatment is optimized is related to the discharge voltage, sinusoidal or pulsed, as the case may be. Optimized pressure is proportional to voltage measured on load. In this connection, a pulsed voltage has been found desirable for film surface treatment with pulse voltage on load exceeding 20,000 volts, while a sinusoidal voltaye performs satisfactorily for treatment of the porous and woven fabric materials with load voltages of 1000 to 3000 volts proving adequate and it will be noted 2600 volts were utilized for Examples 4-8, Table II.

Power applied (and its relationship to treatment time) Series A, Examples 1-3.-A series A of Examples 1-3, set forth in Table I, show that treatment time and discharge power are inversely proportional. For each of these examples the flow rate of the treatment gas was 5.4 millimoles of CE, per minute at a pressure of 5 torr. The apparatus of FIG. 1 was used. The treatment times used in most examples are determined primarily by the characteristics and capabilities of the electrical power supplies used. The energy required per unit of surface treated to the present optimized state has been found to be within the accuracy of the measurement 0.2 k.w.h./ yard for a variety of surfaces studied including cotton, wool, paper and the synthetic fabrics. Examples shown in Table IV applied 0.2 k.w.h./yard 2 and it will be noted that Example 14 achieved a Water repellency test substantially better (i.e., 20.8%) than is shown in the following Table I which reports examples utilizing only about 0.07 k.w.h./yard Treatment time Series B, Examples 4-8.In a series B of Examples 4- 8 involving treatment times of from 25 to 400 seconds a span 4 of cloth to be treated with the apparatus of FIG. 1 (bleached and desized 80 x 80 weave cotton) was wound on the supply roll 2 and the end threaded through the guide slits 6, 7 and between the upper electrode array 8 and the lower electrode array 9 and attached to the take-up roll 3. (A series F of Examples 54-91 described later, will involve treatment times as short as 3 seconds.)

The treatment chamber 1 was sealed and evacuated via outlet port 12 using a Welch Model 1397 mechanical pump. After evacuation to below millitorr absolute the electrical terminals 21 of the electrodes 8a and 9a were connected to the described source of high voltage continuous wave radio frequency power. For the series B, Examples 4-8, CF gas was admitted to the chamber via the inlet port 14 and its 'fiow directed over and under the cloth via the gas flow holes 18, 19 (FIG. 2) in the electrode support panels 10, 11.

The GE; gas pressure was read on a gauge connected to pressure gauge port 16 and adjusted by means of throttling outlet port 12 to the pressure of 5 torr after setting the flow rate at the valve of the gas supply cylinder (not shown) to a value of 5.4 millimoles/min.

The area of cloth to be treated was moved into the space between electrode arrays 8 and 9, and the electrodes were subjected to an electrodeless discharge of 585 watts peak power at 2600 volts peak to peak measured at reaction chamber terminals 21, for the length of time indicated in the following Table II. Table II shows how in the treatments of Examples 4-8, series B, the amount of water absorbed was reduced compared to an untreated control sample of the same cotton cloth and the effect of treatment times on the results obtained.

Pretreatment wash Series C, Examples 9-13.The materials used for Examples 9-13 were samples of the same bleached and desired -80 weave cotton cloth used for series B, Examples 4-8. In Examples 9-13 the samples were first thoroughly Washed in distilled water to remove residual chemicals from desizing after which the same steps were followed in the treatment processing and testing as for series B, Examples 4-8. The following Table HI shows how inclusion of a washing step in the treatment processing of series C examples made the material even more water repellent than the treatment processing of series B as shown in Table II and lowered considerably the length of treatment required to achieve a given degree of water repellency as measured by the immersion test.

TABLE III Water repellency (static immersion test) Treatment time, sec.

Series B,

Example No. percent Nature of the treatment gases 14-49 the treated cotton samples were tested for water repellency using the water drop test. After treatment processes with 100 second treatments, i.e., Examples 14, 18, 22, 26 and 30, the treated cotton samples were also tested for water repellency using the static immersion test. For each of these Examples 14-49 the results shown in the following Table IV demonstrate that optimized treatment is (a) restricted to the perfluorocarbon gases, i.e. perhalohydrocarbons having the C-X bond greater than 100 kilocalories (where X=halogen), (b) the presence of hydrogen in a treatment gas molecule decreases significantly the efficiency of surface treatment achieved, the presence of chlorine results not only in a decrease in efliciency (longer treatment time required) but in a significant decrease in the maximum amount of water repellency which could be obtained for any treatment time used and (d) the use of N 0, an oxidizing agent and electron receptor resulted in no treatment as measured by the water repellency tests.

TABLE IV Treatment time (seconds) 100 50 25 100 sec. sec. sec. sec. sec.

Water repellency static immersion test, percent Water repellency water drop Series D (Control= test, rs. (Contr01= 1-3 Exam. No. Gas 56.8%) seconds) As appears from examples described, treatment gases are those containing the fluorine atom, with perfiuoro compounds such as CF4, C2F6, C4F3( C3F8, and SP6 the most satisfactory. The presence of hydrogen as in CHF in the treatment gas results in decreased efiiciency and the presence of chlorine as in CCl F reduces the amount of treatment achieved to almost negligible amounts. Gases such as tetrafluorethylene, hexafluoropropylene, are conceived as being good treatment gases. However, because of their unsaturation, polymerization as well as chemical replacement occurs and the polymerization reaction predominates. Fabrics treated in an atmosphere of C F quickly lose their hand and suffer loss of their air-breathing property. Similarly, reagents such as BF and SiF are conceived as being good treatment gases. However, because of their rapid chemical reaction with even traces of moisture to yield the white powdered products, H BO and H SiO respectively, their use in a process requiring frequent opening of the reactor would result in contamination of the reactor and of treated porous materials with the H BO or H SiO and HF produced in their reaction with the moisture in the arr.

Materials treated-textile fabrics other than cotton Series E, Examples 50-53.Examples 1-49 all involve cotton and Examples 50-53 of Series E, involve treating a variety of textile fabrics other than cotton, according to the present invention. For these examples the treatment gas was CF at a pressure of 5.0 torr and with a flow rate of 5.4 millimoles for treatment times of 50 and 100 seconds as indicated in Table V, which were also the conditions for the cotton Examples 5 and 6 of series B set forth in Table I I. In the following Table V the Examples 5 and 6 are repeated for convenience in compar- 10 ing with cotton the examples processing wool, rayon, nylon and Dacron.

TABLE V Voltage Water Treat- (max. repellency, ment peak static Exam e time to peak Power immersion No. Fabric (sec.) volts) (watts) test, percent 10.8 14.6 nylon.

Cot 2,600 585 27.0 Control-.- Untreated 56.8

52 Wool.-..'.-'.: 100 2,600 310 21.3 Contro Untreated 45.8

wool.

53 Dacron 100 2,700 560 14.8 Control Untreated 19.3

dacron.

It is seen in the data of Table V that significant improvements in water repellency are obtained for all fabrics treated and that even nylon, which is relatively water repellent before treatment, is improved by treatment.

Materials treated-cellulose-paper Series F, Examples 54-91.Table VI to follow discloses the data obtained from a series F of Examples 54-89, Table VI discloses data from Examples 89-90", and Table VIII discloses data from an Example 91.

In Examples 54-91 the surfaces of a 5 mil thick Dryden kraft paper of intermediate porosity were treated with CE, treatment gas at pressures from 0.5 to 300 torr and with treatment times of 5 to 50 seconds for the examples in Table VI and of 100 seconds for the examples in Tables VII and VIII. It Was noted that a well dried paper yields a significantly more effective treatment. The data indicate that pressures of CE, up to 300 torr result in satisfactory treatment in the treatment time range of 25 to 50 seconds at the power level used.

The data of Table VI also shows that satisfactory treatment may be obtained with shorter treatment if the power is raised sufiiciently. It can be noted that Examples 64-68 demonstrate that with a pressure of 50 torr and relative discharge power 1.0 (the absolute discharge power being 0.2 kw./yd. that effective treatment was not obtained with treatment times less than 25 seconds. However, Examples 84-88 show that increasing the power provided effective treatment with treatment times as short as 3 seconds.

TAB LE VI Relative treatment times in seconds. water Gas disdrop test in hours pressure charge (torr) power 50 sec. 25 sec. 12 sec. 6 sec. 3 sec.

0. 5 1. 0 6. 00 3. 50 1. 50 5. 0 1. 0 5. 25 5. 00 5. 25 4. 25 4. 25 50. O l. 0 7. 00 4. 25 100. O 1. 0 7. 25 7. 25 6. 00 6. 00 0. 50 200. 0 1. 5 5.00 5. 00 300. 0 2. 0 5. 50 5. 50 5. 25 5. 00 0. 16 50.0 1. 5 7. 00 6. 25 5. 25 4. 25 1. 50

1 No test made.

Since Tables 11 and III indicated that little gain was achieved by extending treatments beyond 100 seconds, the conditions of Examples 59-63 (except for time) were repeated for 100 seconds as Examples 89 and 90 and results are shown in the following Table VII.

1 Le. untreated krait paper;

Tables VI and VII therefore show by Examples 54-90 of the present invention, that water repellency of kraft paper as measured by the water drop test has been increased from a matter of 1 to 3 seconds, to a matter of several hours; and as measured by the static immersion test, the water repellency of kraft paper has been doubled.

Samples of the 5 mil thick Dryden kraft paper of intermediate porosity were treated using the gas CF Treatment time was 100 seconds and the discharge power was 650 Watts at a load voltage of 2.3 One-half inch strips, eight inches in length, were cut from the treated paper and from untreated paper and the tensile strengths measured with an Instron Model IT-C Universal Testing apparatus, in air, and with the use of auxiliary liquid immersion test equipment, under a 6-12 inch head of distilled water. The results are given in Table VIII.

It is seen from the data in Table VIII that no significant change in dry paper tensile strength has resulted from the treatment according to this invention. However, upon comparison of the tensile strengths measured under water it is shown that the treated samples retain approximately one-third of their dry tensile strength while the untreated control fell to a value of of the average dry strength for a net gain in wet tensile strength of 10 for the treated paper.

Materials treated-polyethylene Series G, Examples 92.-105.Examples will now be given of the present invention applied to tubular samples polyethylene treated with the apparatus shown in FIG. 3. Samples so treated will be tested to measure resistance to corrosion in the manner described in the section entitled Tests and Measurements. This approach has been chosen because polyethylene as a tubular sheath has been found to be particularly useful to protect wires and cables for underground burial. In this use for polyethylene it is known that the main factor contributing to degradation of the electrical insulating properties of the polyethylene coaing is oxidative degradation (Underwater Degradation of Polyethylene for Wire Insulation a paper presented by Yoshida, Tsunemi, Hasebe, Morikuni and Fukuda, Teruo at the Fifteenth Annual Wire and Cable Symposium, Atlantic City, New Jersey, U.S.A., Dec. 8, 1966).

Example 92.A 2 cm. long section of ID, /8" OD. polyethylene tubing was placed on the combination electrode/sample holder tube 36 in the cell 30 (FIG. 3) and the cell evacuated via the outlet port 32. Helium gas was introduced into the cell 30 through port 31 at a constant slow flow rate and at a pressure of 0.12 torr. The two electrodes 35, 40 were connected to the output terminals of a Cenco No. 80700 Tesla coil powered by a Cenco No. 80365-3 high voltage transformer which in 12 turn was energized by a 60 Hz., 28 v. RMS source. The resulting discharge containing pulses of up to 35 kv. was continued for a total of 8 minutes with the sample holder 36 and sample 37 rotated one revolution during this time to insure uniform exposure of the outer surface of the sample to the discharge.

Following this pretreatment, the helium gas was turned off and the CE; treatment gas introduced at a pressure of 0.5 torr and substantially the same flow rate. A discharge of essentially the same characteristics was carried out in the CR, environment for a total of 20 minutes with the sample holder 36 and sample 37 again rotated one revolution during this time to assure uniform exposure of the sample surface.

After treatment the sample 37 was removed from the treatment cell and oven dried to constant weight, along with an untreated control and a similar sample of the premium price polytetrafluoroethylene (Teflon). The weighed sample, control and polytetrafluoroethylene were then immersed in the oxidizing, corrosive environment provided by chromic acid solution at a concentration typically used for cleaning glassware, 150 ml. conc. H SO +40 g. Na Cr O at il C. for .6 hours, following which the sample and control were again dried and weighed. For tubular samples weighing 700:20 mg. and an exposed area of 6 cm. the following results were obtained as set forth in Table IX and plotted in the graph shown as FIG. 4.

Example 93.-Two samples of tubing, as used in Example 92, were treated in the manner described in Example 92 with the single exception that C F was substituted for CE; as the treatment gas to indicate the feasibility of using other fluorocarbon gases. The following results were obtained as set forth in Table X.

TABLE X Wt. loss, mg. Treated polyethylene sample 5.7 Treated polyethylene sample 5.8

Control (untreated polyethylene) 18.3

After 6 hrs. immersion in chromic acid at 75 0.

Example 94.-A 2 x 2.5 cm. sample of 6 mil polyethylene film was wrapped around the sample holder 36 of the cell 30 and subjected to the same treatment described in Example 92. The film was then removed and rewrapped on the holder with the opposite side of the film exposed to the discharge region. This side was then subjected to the same treatment. The sample was then dried, weighed and tested in 751-1 C. chromic acid giving the results shown in the following Table XI.

TABLE XI Wt. loss, mg. Treated polyethylene sample 2 3.1 Control (untreated) 3 20.5

After 6 hrs. immersion in chromic acid at 75 C. 2 Total film Weight: 60.8 mg. 8 Total film weight: 63.2 mg.

A series of samples of 6 mil polyethylene film were treated in accordance with the procedure of Example 94 and along with similar untreated samples were analyzed for fluorine content by the method described under the heading Fluorine Analysis in the section entitled Tests and Measurements. A plot of the results obtained showing fluorine content vs. surface area is shown in FIG. 5.

These results show that the treatment fluorinates the sur- A TABLE XII Helium dis- Wt. loss due to charge time, corrosion in Example min. chromic acid,mg.

95 4 13. 5 (Example 92).-. 8 2. 5 96 16 2. 4

Examples 97-99.Three samples of polyethylene tubing as described in Example 92 were treated in accordance with the procedure of Example 92 except that the helium pressure used during the helium discharge was varied. The following results shown in Table XIII were obtained which show that low pressure is required for the pretreatment step. The amount of weight loss due to corrosion is seen to increase sharply at the highest of the three pretreatment pressures used.

TABLE XIII Helium dis- Wt. loss during charge preschromic acid Example sure, torr. corrosion, mg.

Examples l00102.-Three samples of polyethylene tubing treated and tested according to the procedure described in Example 92 except that the time of exposure to the OR; discharge was varied. The following results shown in Table XIV were obtained showing the effect of the treatment time in the fluorocarbon gas discharge.

TABLE XIV Treatment Wt. loss during time to CF 4 chromic acid Example discharge, min. corrosion, mg.

Examples l03-105.Thre'e samples of polyethylene tubing were treated and tested in accordance with the procedures described in Example 92 with the exception that the pressure of the CE; gas during the discharge was varied. The following results shown in Table XV were obtained illustrating the eifect of variation in the fluorocarbon gas pressure.

TABLE XV CFi discharge Wt. loss during pressure, chromic acid Example torr. corrosion, mg.

14 MECHANISM OF THE INVENTION The mechanism operative in achieving the surface changes described and illustrated in the examples which have been described is describable by a series of chemical steps initiated by the discharge. Although it is not intended to restrict this patent by mechanism, it is felt that the consistency of the data with the explanation of mechanism to follow together with the difference in kind of the changes which are observed to serve to describe the present invention as well as to distinguish it from the inventions and changes achieved in the processes of others, e.g., Wolinski and Coleman which have been discussed under the Description of the Prior Art.

The mechanism, in brief, visualized the activating of a gaseous species containing a desired reactant, for example, the fluorine atom in the presence of the surface with which it is to react. If the surface is sufliciently reactive, the reaction is carried to the limits permitted by reaction equilibria and the surface properties desired.

For the special and somewhat unusual case of a relatively inert surface such as polyethylene, pretreatment of the surface by discharge in helium gas at a pressure low enough to permit eifective ion bombardment (and activation of the surface) precedes treatment in the reactant gas. The low gas pressure requirement observed for pretreatment is believed to reflect the need for an average mean free path large enough to insure ion energies capable of rupturing chemical bonds. Polyethylene may be taken as an example of an inert surface requiring both pretreatment and treatment.

Pretreatment in helium at 0.1 torr (1) Discharge ionization of the helium He s;/ He+ e- (2) Activation of the polyethylene sites by energetic helium ions i iI He+ 116 0* l x I i where *=E (energy sufficient to cause rupture Treatment in CF at 5 torr (1) Discharge activation of CF;

(2) Reaction of discharge species with active polyethylene sites l x A) CF;

on c l x x For the case of the more reactive surfaces such as, for example, the cellulosics, cotton or paper, the following equations serve to account for the observed behavior: Treatment in CF (1) Discharge activation of CE;

(9.) "OH H l n la +3. m

Test of mechanism Saturation of treatment.To distinguish between chemical reaction with the surface as illustrated in the above equations and discharge initiated polymerization onto the surface, the following experiment was devised. A series of 80-80 weave cloth samples were selected to be as uniform as possible and were discharge treated in CF, for varying lengths of time over a significantly Wider range of treatment time than was used in the treatment optimization study. The samples were then analyzed for fluoride content. If the prevailing treatment reaction is replacement of a surface atom by F or CF saturation of the surface with F should be reached for the samples treated for the excessive time periods and a plot of fluoride content versus treatment time should fall off from a linear relationship. Conversely, if polymerization occurs to any significant extent, no saturation with treatment time should occur and the amount deposited should be deposited linearly in time. Parameters such as discharge power and gas pressure were maintained constant.

The results, FIG. 6, show the rate of the fluoride content addition to the cotton samples to fall off rapidly after '16 seconds of treatment and remain almost unchanged after 250 seconds indicating that deposition by polymerization is very limited or completely absent.

Correlation between calculated fluoride content of saturated (perfluorinated) polyethylene surface and the fluoride found in analyzed samples treated according to Example 94 Polyethylene films, 6 mils thick, and having macro surface areas of 4 square centimeters were discharge treated in CR, on both sides as described in Example 94 were analyzed for fluoride using a modification of the method of Fluorine Analysis which has been described. The results, plotted in FIG. 5, show a fluoride content of the treated film to be linear in treated area. After correction for a low concentration of fluoride in the untreated polyethylene, discharge applied fluoride is calculated from FIG. 5 to be 2.79 micrograms of fluoride per square centimeter of treated surface.

One can with the help of some simplifying assumptions calculate'an approximate number of replaceable surface hydrogen atoms on a polyethylene surface area for comparison with the observed fluorine atoms found per unit of macro area. If one assumes a polyethylene micro surface having an average of 1.25 angstrom (10- cm.) distance between carbon atoms, based on a 154x10 CC bond distance and an average bond angle of accessibility of treatment to all surface hydrogen atoms, and for simplicity, uniformity of carbon atom array in the surface plane, one can calculate the potentially replaceable hydrogen atoms as of micro surface assuming two replaceable CH bonds per carbon atom. An increase of a factor of two to three over this value calculated fora smooth surface can easily be expected depending on the ratio of micro to macro surface. The value observed 2.79 micrograms/cm. for fluorine content of the surface-treated films corresponds to a value of 19 =8.8 10 atoms/cm.

6 l0 =atoms/mole and 19=grams/atom mole for fluorine, a value quite consistent with the value of 2.6 10 atoms/cm. calculated fromamodel which assumed the replacement of surface H atoms by F atoms.

The assumptions made for the foregoing calculation were intentionally and to a degree necessarily oversimplified. Assumptions of a flat surface and of uniform C-C bond distances in both directions in the surface plane, for example, are oversimplified, but the errors introduced by their assumption would be compensatory and of the same approximate magnitude. Similarly, the assumptions of complete teflonation, that is 100% conversion of all accessible CH bonds to CF or CCF bonds and that only the surface is fluorinated lead to presumably smaller and compensatory errors.

One can therefore expect to predict to within at least an order of magnitude the fluorine-content per square centimeter of treated polyethylene film and in particular shoud be able to differentiate a surface perfluorination reaction from a polymerization reaction onto the surface. It has been here shown that the calculated value for F atoms/cm. of surface is 2.6x 10 which compares very well with the 8.8 10 atoms/cm. found upon chemical analysis of a large group of the treated films. The fact that the measured value is greater than the calculated value is felt to reflect the oversimplified assumption of fiat surface. In fact, the microsurface area is 17 significantly larger than the assumed macrosurface area due to the surface roughness.

By the foregoing description of apparatus, process steps and resulting materials of manufacture, it has been shown that by the present invention a treatment process has been found which increases the stability of polyethylene to oxidative attack. A 7-fold decrease in Weight loss of treated samples was observed after 6 hours in chromic acid at 75 C. Weight loss of treated samples was 0.47 mg./cm. compared to 3.4 mg./cm. for untreated polyethylene for comparable 6 hour exposures. It has also been by the foregoing description that the present inven tion is applicable to a wide range of natural and synthetic polymers in woven or porous sheet form. As expected for the case of perfluorination, increase in water repellency has been found to be a general property change. The water absorption of cotton cloth with a natural water absorption, (ASTM D58363), of 57% has been reduced to 15-20%. Water absorption of wool cloth has been reduced from 58% to 22%. Treatment of nylon, which is already water repellent, decreased its water absorption from 15% to 11%. Treatment of kraft paper resulted in a reduction of its natural water absorption from 82% to 39% and an increase in wet tensile strength from 280 lbs/in. to 3000 lbs./in.

These observed desirable changes in properties have been related to the addition of fluorine or fluorine-containing groups to the surfaces in question and new materials having the aforesaid desirable characteristics have been thereby created. Microchemical analysis of polyethylene film surfaces which had been discharge treated and shown to possess seven times the corrosion resistance of untreated film were found to contain 2.79 micrograms of fluorine per cm. of surface. Similarly, analysis of treated 80-80 weave cotton cloth (water absorption=20%) showed a fluorine content of 21.5 micrograms per square centimeter of macro surface. In general,

fluorine content has been shown to be linear in discharge treatment time at short treatment times.

From the foregoing detailed description of the present invention, it has been shown how the objects of the invention have been attained in various preferred manners. However, further modifications and equivalents of the disclosed concepts such as readily occur to those skilled in the art are intended to be included within the scope of this invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for increasing the corrosion resistance of polyethylene including the steps of: arranging polyethylene between electrodes separated by an insulating material; flowing helium gas between the electrodes to contact the polyethylene; applying electric power to the electrodes to subject the polyethylene surface in the flowing helium gas to an electrodeless discharge of electrical energy until the inert surface of the polyethylene is converted to a chemically active surface; selecting a gas from a group consisting of CF C F C 1 C 1 CHF and SP flowing the selected gas in contact with the activated surface of the polyethylene; and applying electric power to the electrodes to subject the polyethylene in the flowing selected gas to an electrodeless discharge of electrical energy unitl the selected gas is chemically activated and hydrogen in the surface of the polyethylene is replaced by fluorine from the selected gas to thereby provide a fluorinated surface on the polyethylene.

2. A process according to claim 1 in which the helium is at a pressure between about 0.06 torr and 0.24 torr.

3. A process according to claim 1 in which the surface activation time of the polyethylene in the helium gas is between about 4 minutes and 15 minutes.

4. A process according to claim 1 in which the selected gas is at a pressure between about 0.25 torr and 1.00 torr.

5. A process according to claim 1 in which the period of time during which the polyethylene is subject to the discharge of electrical energy in the selected gas is between about 10 and 40 minutes.

6. A process according to claim 1 in which the surface of the polyethylene is activated in helium gas at a pressure between about 0.06 torr and 0.24 torr for a period of time between about 4 minutes and 16 minutes; the selected gas is CF, at a pressure between about 0.25 torr and 5.00 torr; and the polyethylene is subject to the discharge of electrical energy in CR, gas for between 10 and 40 minutes with the discharge containing pulses up to 35 kv. to fluorinate the surface to the extent of about 2 milligrams of fluorine per cm? of surface area of macro surface.

References Cited UNITED STATES PATENTS 3,069,286 12/1962 Hall 204-169 3,252,830 5/1966 Cummin et al. 117-217 3,255,099 6/1966 Wolinski 204-169 3,275,540 9/1966 McBride 204 3,485,734 12/1969 Pecka 204-465 3,494,845 2/ 1970 Osborn et al 204-169 3,526,583 9/1970 Haywood 204-165 3,600,122 8/ 1971 Coleman 204-165 F. C. EDMUNDSON, Primary Examiner US. Cl. X.R.

ll7-93.1 CD, 93.4 NC; 204165, 168 

